Interface structures for electronic devices

ABSTRACT

An interface includes a surface having an electrically conductive pad; a compliant coating over the surface having a via extending to the pad; metallization patterned over the compliant coating and extending into the via; a low modulus dielectric interface layer overlying the compliant coating and having an interface via extending to the metallization; and a floating pad structure including floating pad metallization patterned over the dielectric interface layer with a first portion forming a central pad and a second portion forming an extension from the central pad extending into the interface via. Another interface includes a substrate including a low modulus dielectric interface material having a hole extending at least partially therethrough and a floating contact structure including electrically conductive material coating the hole with at least some of the floating pad metallization forming an extension from the hole. A conductive contact area interface may include an electrically conductive first contact area; an electrically conductive second contact area facing and being substantially aligned with the first contact area; and at least one interface structure coupled between the first and second contact areas and including an electrical conductor having a partially open interior to form a compliant joint between the first and second contact areas.

BACKGROUND OF THE INVENTION

Ball grid array (BGA) technology provides a high density ofinterconnections per unit area, but mismatches of coefficients ofthermal expansion (CTEs) occur when ceramic or polymer BGA substratesand printed circuit boards are joined and often result in cracked solderjoints, especially as the size of the substrates and temperature rangesare increased. In column grid array (CGA) techniques and other BGAtechniques, a eutectic solder is applied to printed circuit board andmulti-chip module array pads and the resulting joint is soldered to ahigher temperature solder column or ball which does not melt. Both BGAand CGA structures can be inflexible and vulnerable to damage. Forvarious types of BGA and CGA, increases in reliability are attempted byelaborate under-filling of the structures with polymer glues toreinforce the interfaces and reduce the effects of the CTE mismatch onthe solder joints. The polymer glues, however, impair repairabilitybecause of the difficulty in removing the glues after hardening.Furthermore, these types of structures require two separate soldersteps, are more expensive than conventional solder structures, andrequire more vertical space due to increased height of the joints.

One conventional micro ball grid array interface technique for attachinga semiconductor circuit chip directly to a substrate is to use a seriesof solder bumps clustered at the center of the chip to constrain thearea over which stresses between differing coefficients of thermalexpansion occur. In this embodiment, chips have their pads reconfiguredand solder micro bumps are applied over the reconfigured pads. In oneembodiment, ball grid array processes are used with the temperaturerange being constrained during device operation to 30° C. to 70° C. inan effort to avoid CTE stress effects. In another ball grid arrayinterface technique, the area where the chip faces the printed circuitboard or substrate is not used for direct interconnection. Instead,metallization is routed from the chip to adjacent support structureswhich then have solder ball connections. This technique can create sizeand pin count limitations as well as electrical parasitic effects.

SUMMARY OF THE INVENTION

It would be desirable to have a method for providing highly compliantelectrically conductive interconnections for structures having differingcoefficients of thermal expansion and to have a base pad andmetallization contact area with long term reliability (i.e. withoutcracks or breaks) even under thermal and material stress conditionswithout the need for rerouting chip pads to the center of a chip or toan adjacent support structure.

In one embodiment of the present invention, a method and structureelectrically interconnect materials having different coefficients ofthermal expansion. A "floating pad" structure is used to increasereliability by providing stress and thermal accommodation of the twomaterials and permitting movement of the floating pad independent of thebase pad. The invention includes a floating pad interface structure thatis connected to a semiconductor chip's original pad by means of microextensions that provide stress relief for different coefficients ofthermal expansion. The floating pad interface structures can include asingle pad and extension or several extensions in situations wherein asingle extension is not sufficient for extreme thermal stress/strainsituations. The present invention provides a structure that accommodatesthermal and material stresses without submitting the via interconnectareas to forces that can crack vias or break connections at the chippads. The floating pads permit movement independent of a semiconductorwhile providing electrical interconnections through selected materialsthat are specifically patterned to provide low forces at the via areasand thus accommodate differential thermal stresses which may be causedby large CTE differences.

In another embodiment of the present invention a micro structureinterface is provided that is solderable and forms an electronicinterconnection without requiring pressure. The interconnections can beheld in position prior to application by an interposer that providesease of assembly and surface mount technology self-alignment capability.A conductive contact area interface may comprise: at least oneelectrically conductive first contact area; at least one electricallyconductive second contact area facing and being substantially alignedwith the at least one first contact area; and at least one interfacestructure coupled between the at least one first contact area and the atleast one second contact area. The at least one interface structurecomprises at least one electrical conductor having a partially openinterior to form a compliant joint between the at least one firstcontact area and the at least one second contact area.

BRIEF DESCRIPTION OF THE DRAWINGS

The features of the invention believed to be novel are set forth withparticularity in the appended claims. The invention itself, however,both as to organization and method of operation, together with furtherobjects and advantages thereof, may best be understood by reference tothe following description taken in conjunction with the accompanyingdrawings, where like numerals represent like components, in which:

FIG. 1 is a sectional side view of a compliant coating overlying a basepad.

FIG. 2 is a view similar to that of FIG. 1 further including a firstinterface dielectric layer.

FIG. 3a is a view similar to that of FIG. 2 further including floatingpad structures overlying the first interface dielectric layer.

FIG. 3b is a view similar to that of FIG. 3a further showing the removalof the first interface dielectric layer.

FIG. 4 is a top view of one of patterned floating pad structuresoverlying the base pad.

FIGS. 5a, 5b, and 5c are top views of other floating pad embodiments.

FIG. 6 is a sectional side view of another floating pad embodiment.

FIG. 7 is a view similar to that of FIG. 3a further including a secondinterface dielectric layer and a second pair of floating pad structuresoverlying the first interface dielectric layer.

FIG. 8 is a top view of one of the second pair of floating padstructures.

FIG. 9 is a view similar to that of FIG. 7 further including a thirdinterface dielectric layer having openings over the second floating padstructures for positioning of a solder bump and a solder ball andshowing excess dielectric material removed.

FIG. 10 is a view similar to that of FIG. 3a further showing a secondinterface dielectric layer having vias with metallization for attachmentof a solder bump and solder ball.

FIG. 11 is a view similar to that of FIG. 3a further showing a secondinterface dielectric layer having vias in which solder is present.

FIG. 12 is a view similar to that of FIG. 11 further including a thirdinterface dielectric layer having openings over the solder forpositioning of a solder bump and a solder ball.

FIG. 13 is a sectional side view of an interface structure for afloating pin contact.

FIG. 14 is a top view of one of the pins of FIG. 14.

FIG. 14a is a sectional side view of an interface structure whereinholes do not extend all the way through an intermediate substrate.

FIG. 15 is a sectional side view of a printed circuit board and amultichip module each having area array pads.

FIG. 16 is a view similar to that of FIG. 15 further including severalinterface embodiments of the present invention.

FIG. 17 is a view similar to that of FIG. 16 further including aninterposer embodiment of the present invention.

FIG. 18 is a schematic side view of the interface and the interposer.

FIG. 19 is a view similar to that of FIG. 18 further includingsolderable surfaces on the interface.

FIG. 20 is a view similar to that of FIG. 17 showing a curved base.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a sectional side view of a compliant coating 14 overlying abase surface 10 having conductive pads 12. The base surface may comprisea semiconductor wafer that has not yet been cut into segmentedindividual chips or a chip which has been segmented from a wafer.Processing on a wafer that has not yet been segmented enables massfabrication of interface structures and prevents single chip handlingproblems such as corner edge separations of the compliant coating andalignment difficulties associated with single die handling forphotolithographic processing. The base surface may alternativelycomprise, for example, a passive component, a printed circuit (PC)board, a multi-chip module (MCM), a flexible interconnect layerstructure such as described in Cole et al., U.S. Pat. No. 5,527,741,Jun. 18, 1996, or a substrate or wafer including photonic structures,liquid crystal structures, or microelectromechanical structures (MEMS)such as described in commonly assigned Ghezzo et al., U.S. Pat. No.5,454,904, issued Oct. 3, 1995, for example. MEMS and photonic devicescan be directly integrated into wafer processing steps to create anintegrated structure, or MEMS can be used to fabricate a separatestructure which can be later added to the wafer. Conductive pads 12 maycomprise pads or metallization on any of the above-discussed basesurfaces. With the present invention, interface connections forconductive pads are more stable, so the conductive pads can have smallerareas (such as a diameter or length ranging from about one mil to aboutfour mils) than conventional pads.

Compliant coating 14 comprises a material such as a polyimide or asiloxane polyimide epoxy (SPI/epoxy) described by Gorczyca et al., U.S.Pat. No. 5,161,093, issued Nov. 3, 1992. The compliant coating maycomprise a high or low modulus insulative material and typically has athickness ranging from about two micrometers to about 100 micrometers.The compliant coating can be laminated onto base surface 10 with heatand/or an adhesive (not shown) or deposited on the base surface by aspin, spray, or chemical vapor deposition (CVD) technique, for example.

Coating vias 16 are formed in compliant coating 14 by any appropriatemethod. In one embodiment, as described in Eichelberger et al., U.S.Pat. No. 4,894,115, issued Jan. 16, 1990, the compliant coating can bescanned repeatedly with a high energy continuous wave laser to createvia holes of desired size and shape. Other appropriate methods include,for example, photopatterning photopatternable polyimides and using anexcimer laser with a mask (not shown).

Base metallization 18 can be formed by sputtering and/or plating, forexample, and patterned with a standard photoresist and etch process.Eichelberger et al., U.S. Pat. No. 4,835,704, issued May 30, 1989,describes a useful adaptive lithography system for patterning themetallization, for example. The base metallization in one embodimentcomprises a thin adhesion layer of 1000 Å sputtered titanium, coated bya thin layer of 3000 Å sputtered copper, coated by a layer ofelectroplated copper to a thickness of four microns, for example. Anoptional buffer layer of 1000 Å of titanium can be applied over theelectroplated copper. Other example base metallization materials includemolybdenum, tungsten, and gold. The appropriate material of the basemetallization will vary depending on the material of the base surfaceand on the environment, such as a high temperature environment or anoxidizing environment for example, in which the electronic device willbe used.

FIG. 2 is a view similar to that of FIG. 1 further including a firstinterface dielectric layer 20 which preferably comprises a flexible lowmodulus material, such as SPI/epoxy, other flexible epoxies, siliconerubber materials, TEFLON™ polytetrafluoroethylene (TEFLON is a trademarkof E.I. du Pont de Nemours and Co.), or other polymers that have a lowmodulus or have been modified to obtain reduced modulus, having athickness ranging from about two micrometers to about 3 mils. Apreferred thickness range is from about 100 micrometers to about 400micrometers.

First interface dielectric layer 20 has vias 22 therein extending toportions of base metallization 18 overlying compliant coating 14. Vias22 of FIG. 2 can be formed in a similar manner as vias 16 of FIG. 1. Ifthe first interface dielectric layer has a thickness less than that ofthe compliant coating, vias 22 can be made smaller than vias 16, asshown.

FIG. 3a is a view similar to that of FIG. 2 further including floatingpad structures 30 and 30a overlying first interface dielectric layer 20,and FIG. 4 is a top view of one of the floating pad structures 30overlying the base pad. The floating pad structures accommodate thermaland material stresses without imposing undesirable forces on basemetallization areas.

Metallization for floating pad structures 30 can be applied andpatterned by techniques similar to those discussed with respect to basemetallization 18 of FIG. 1. In FIG. 4, the floating pad structuresinclude a central pad 26 having patterned extensions 28 extendingthrough via metallization 24 to base metallization 18.

The size of central pads 26 will vary according to the specific planneduse of the floating pad structure. For example, if a solder ball orsolder bump will be attached directly to the central pad, the centralpad must be large enough to accommodate the attachment. The size of thecentral pad will also affect the length of extensions. Smaller diametercentral pads require less space than larger diameter pads and thereforeleave more space for longer extensions. The area of base metallization18 is an additional factor which can affect the available length ofextensions.

The thickness of metallization for the floating pad structures can beuniform, as shown by floating pad structure 30, or variable, as shown byfloating pad structure 30a. A modified central pad 26a in FIG. 3aincludes central pad 26 and an additional metallization area 27. In oneembodiment, the extensions 28 have a thickness ranging from about 2microns to about 8 microns, and the modified central pad 26a has athickness ranging from about 4 microns to about 20 microns. Thisembodiment is useful because thin extensions are more flexible thanthicker extensions whereas the central pad is preferably sufficientlythick to be solderable to another electrically conductive surface. Inone embodiment the additional metallization area comprises a metal whichis readily solderable such as a nickel-gold plate, for example.

FIG. 3b is a view similar to that of FIG. 3a further showing the removalof the first interface dielectric layer. In some embodiments such asMEMS, for example, removal of first interface dielectric layer 20 isdesirable so that the first interface dielectric layer does not impedethe movement of MEMS devices or interfere with optical devices. Removingthe first interface dielectric layer is also useful because it providesopportunities to fabricate structures having movement that can be usedfor measurement or control purposes. In these embodiments, the materialof first interface dielectric layer 20 is chosen to be a material thatcan be removed by a sublimation process, a solvent, or a laser, forexample, without interfering with compliant coating 14.

In a related embodiment at least a portion of each area 20a under arespective floating pad structure of FIG. 3a can be removed with theremainder of interface dielectric layer 20 being left in position. Ifdesired, the first interface dielectric layer may comprise aphotosensitive material that is sensitized in regions other than in thepad regions and can thus be easily removed from under the floating padinterfaces. This embodiment provides a capability of inserting anapplication specific filler material (which can be solid or fluid) underthe floating pad interface in areas 20a to provide special dampening(vibration) control or thermal management.

FIGS. 5a, 5b, and 5c are top views of other floating pad embodiments.The patterning of the floating pad structure metallization shown in FIG.4 was shown for purposes of example only. Any number of extensions (oneor more) can be used, and the extensions need not be straight. As shownin FIG. 5a, serpentine shaped extension 28a, spiral extension 28b,saw-tooth extension 28c, and bent extension 28a represent potentialshapes for purposes of example. As shown in FIG. 5b, central pad 526 canbe surrounded by extensions 528 which extend to a ring 530 which may inturn have additional extensions 532 extending to another ring 534 havingvias 536 extending to base metallization 18 (not shown in FIG. 5b). Asshown in FIG. 5c, extensions 28 can be curved to form a pin-wheel shapedfloating interface pad. The shapes of FIGS. 5a, 5b, and 5c can be usefulfor reducing mechanical stress on the extensions and are especiallyuseful if base surface 10 is not planar. When selecting an extensionshape, consideration of any resulting inductance effects is appropriate.

FIG. 6 is a sectional side view of another floating pad embodiment thatcan be useful for reducing mechanical stress on extensions. In FIG. 6,first interface dielectric layer 20 has a dip portion 20a which can beformed during application of the first interface dielectric layer, canbe etched from the first interface dielectric layer in a separate step,or can be heat-pressed into shape. When metallization for floating padinterface structure 30b is applied, extensions 28 will then have dipsfor stress relief. Additional metallization area 27a can be applied tobring the metallization of the central pad up to or above the level ofthe extensions. Although not shown, the embodiments of FIGS. 3a and 3bare expected to have some natural dips resulting from the application ofthe metallization of the floating pad structures.

FIG. 7 is a view similar to that of FIG. 3a further including a secondinterface dielectric layer 32 and a second pair of floating padstructures 33 overlying first interface dielectric layer 20, and FIG. 8is a top view of one of the second pair of floating pad structures 33.

In this embodiment, each of the first pair of floating pad structures ispatterned to have some extensions 28 coupled through vias 24 to basemetallization layer 18 and other extensions 28a which are not coupled tothe base metallization layer.

Second interface dielectric layer 32 may comprise a material similar tothat of first interface dielectric layer 20 which is patterned toinclude vias 36 extending to extensions 28a that are not coupleddirectly to the base metallization layer. The second floating padstructure 33 likewise can be fabricated with central pads 38 andextensions 40 in a manner similar to the first pair of floating padstructures 30.

Although, for purposes of simplicity FIG. 3a illustrates a single layerfloating interface embodiment and FIG. 7 illustrates a double layerfloating interface embodiment, further layers of floating interfacestructures can be added. Such additional layers are useful for providinggreater stress accommodation when a single or double floating pad layeris not sufficient for extreme cases of thermal stress or strain.Multiple layer interface embodiments create additionalthermal-mechanical isolation because the floating pad structures arefarther away and provide a compound lever and thus more degrees ofphysical freedom.

FIG. 9 is a view similar to that of FIG. 7 further including a thirdinterface dielectric layer 42 having openings over the second floatingpad structures for positioning of a solder bump 44 and a solder ball 48in solder flux 46 and showing excess dielectric material removed in anarea 45. Removing the excess dielectric material helps to eliminatestress buildup over the surface area (which can cause bowing or warping)and provides increased compliance and flexibility. Dielectric materialcan be removed by any of a number of techniques such as a photoresistand etch step, mechanical milling, or laser ablation, for example.

FIG. 10 is a view similar to that of FIG. 3a further showing a secondinterface dielectric layer 32 having vias with metallization 50 forattachment of a solder bump 44 and solder ball 48. Metallization 50 canbe applied in a similar manner as the metallization 18. In oneembodiment metallization 50 comprises a nickel-gold alloy which createsan improved contact for soldering. The application of interfacedielectric layer 32 and metallization 50 can be an alternative to theuse of metallization area 27 (shown in FIG. 3a) to increase thethickness of the central pad contact area.

FIG. 11 is a view similar to that of FIG. 3a further showing a secondinterface dielectric layer 32 having vias in which solder 54 is present.Solder can be applied by techniques such as metal screening, heatreflow, sputtering, or electroplating, for example. In the embodiment ofFIG. 11, second interface dielectric layer 32 may comprise a materialsuch as a polyimide or a solder mask.

FIG. 12 is a view similar to that of FIG. 11 further including a thirdinterface dielectric layer 56 having openings over the solder forpositioning of a solder bump 44 and a solder ball 48. Third interfacedielectric layer 56 may comprise a material such as a polyimide or asolder mask and is useful for maintaining solder 54 during solder bumpor ball attachment. If desired, first, second, and third interfacedielectric layers 20, 32, and 56 can be removed entirely, as discussedwith respect to first interface dielectric layer 20 of FIG. 3b or inselected locations (along with compliant coating 14, if desired), asdiscussed with respect to FIG. 9.

With the present invention, MEMS, photonic devices, liquid crystalstructures, and semiconductor chips such as silicon, gallium arsenide,and silicon carbide chips, for example, can be directly soldered tovarious substrate materials such as ceramics, polymers, and flexibleinterconnect layers, for example, with high reliability over a broadtemperature range. Additionally, repair can be performed by heating andmelting the solder without having the conventional step of reconfiguringchip pads to a central location.

FIG. 13 is a sectional side view of an interface structure for afloating pin contact, such as may be present in a pin grid array (PGA)device, and FIG. 14 is a top view of one of the pins of FIG. 14.Coupling rigid pin connection structures to PC boards can createmechanical stresses during fabrication and operation similar to thosestresses created when bonding semiconductor chip pads to a substrate.Microprocessor chips can create high temperatures which create large CTEdifferentials between the chips and PC boards on which they areattached.

In FIGS. 13 and 14, a substrate 210 may comprise a PC board, a flexibleinterconnect layer, or any other structurally appropriate material. Lowmodulus interface dielectric regions 212 may comprise materials similarto those discussed with respect to first interface dielectric layer 20of FIG. 2. The interface dielectric regions 212 can be coated withmetallization 214 which covers holes in the interface dielectric regionsand has extensions 216 extending therefrom on each surface of substrate210. Preferably, the metallization is patterned to form extensionsextending to a pad interconnect region 211 which is coupled bymetallization runs to other pad interconnect regions or metallizationareas (not shown) of the substrate. Solder 222 can be applied betweenthe metallization and an electrically conductive pin 220 that isattached to a support structure 218. "Floating" through hole structuresresult because the interface dielectric regions and extensions permitthe substrate to adjust to mechanical stresses to differing coefficientsof thermal expansion and mechanical stresses encountered when insertingand removing pin grid arrays and other pin devices.

Although the holes are shown in FIG. 13 as extending through theinterface dielectric regions, the invention is also useful in situationswherein the holes extend only part of the way through the interfacedielectric material.

FIG. 14a is a sectional side view wherein holes do not extend all theway through an intermediate substrate 210a. FIG. 14a also illustrates anembodiment wherein the entire substrate comprises a low modulusdielectric region. The substrate in FIG. 14a is useful as a couplingboard between a PC board 240 and a chip having pins, for example.Intermediate substrate 210a has intermediate pads 242 which can becoupled by internal metallization to pads or other metallization on anopposing surface (not shown) of the intermediate substrate and can becoupled by solder 246 to pads 244 of a PC board 240, for example.

FIG. 15 is a sectional side view of a first surface represented by aprinted circuit (PC) board 110 having first contact areas represented byarray pads 112 which are coated with solder 114 and a second surfacerepresented by a multichip module (MCM) 116 having second contact areasrepresented by array pads 118 which are coated with solder 120 andsubstantially aligned with respective array pads 112.

Although array pads on a PC board and MCM are shown, the presentinvention is useful for coupling any of a number of metal areas. Othertypes of metal contact areas wherein the present invention can be usefulinclude, for example, metallization runs on substrates, metallizedsubstrates, flexible interconnect structures, circuit chip pads,semiconductor wafer chip pads, infra red sensors, and holographicarrays.

If a surface such as PC board 110 is a heat sink, includes a heat sink,or is attached to a heat sink 113 by an adhesive 111 (as shown in FIG.15), the interface structures can create a thermal path that can be usedto remove heat from an MCM.

FIG. 16 is a view similar to that of FIG. 15 further including severalinterface structures of the present invention. Each of the interfacestructures has partially open interior to form a compliant joint betweencontact areas that it couples. "Partially open interior" means aninterior that includes air (open spaces) or a compliant material inaddition to the metal that is used to electrically couple the contactareas.

One such interface structure is represented by a crushed wire assembly122 which can be fabricated by pressing very fine metal wires into astructure having an appearance similar to that of steel wool and whichintrinsically has spring action in several planes. Such wire assemblyinterface structures are available from Cinch Connector Devices, of ElkGrove Village, Ill., under the trademark name of FUZZ buttons. Wirepaths are of lower inductance than BGA and CGA, so the size of powerdrivers for the electronics can be reduced.

Wire assembly 122 may comprise materials such as copper, gold, berylliumcopper, gold-plated beryllium copper, and precious metal alloys such aspalladium gold alloys. Typically the thickness of the wires range fromabout 0.5 mil to about 20 mils. A single compressed wire or a pluralityof pressed wires can be used for a crushed wire assembly. For standarddie pads which range from three to seven mils a diameter of wireassembly 122 would be about 3 mils. Wire assembly 122 can have any shapethat permits coupling of two metal areas. For example, round,cylindrical, hour glass, and rectangular shapes are also appropriate forthe wire assembly.

Another interface structure is represented by a stranded core wireassembly 124 which includes stranded vertical wires surrounded byinsulation. In one embodiment the wires comprise fine copper insulatedwires. The insulation must be a material that can withstand thetemperatures at which soldering will occur.

One example material is TEFLON™ polytetrafluoroethylene. The wires canbe bundled together with insulation and inserted between the two metalareas. If the wires are not exposed at the ends of the interface, asolvent dip, mechanical grinding, or etch process can be used to exposethem. Because of the insulation, solder will not appreciably wick intothe stranded core wire assemblies.

Another interface structure is represented by a stranded core wireassembly 126 having removable insulation comprising a material such as adissolvable polymer or a wire varnish. The use of sublimatableinsulation is analogous to transformer insulating varnish which can beremoved by sublimation or solvents, for example, after soldering iscompleted. As discussed below, when the insulation is removed from thevertical wires, an enhanced flexible structure is created.

Another interface structure is represented by a crushed wire assembly128 having capped ends 129. Crushed wire can have an uneven surfacewhich can be evened out by caps comprising solder or a metal forfacilitating soldering of the interface to the contact areas. Crushedwire assemblies can also include removable insulation or solder resistif desired to further inhibit solder wicking into crushed wires. Suchsolder wicking would reduce the flexibility of interfaces.

Another interface structure is represented by a compliant column 130.Column 130 comprises an outer layer of metallization 131 surrounding aninner area which may comprise a flexible material, one example of whichis a polymer such as a rubber, or a hollow area. To provide a hollowinner area 133, material can be initially present during fabrication andlater removed by a process such as sublimation or diffusion. In thisembodiment one or more openings (not shown) would be needed in the outerlayer of metallization to facilitate inner material removal.

FIG. 17 is a view similar to that of FIG. 16 further including anoptional interposer structure 132. The interposer is useful for holdingthe interface structures in position during soldering. The interposercan be either left in position after soldering or removed. If theinterposer structure will be removed, a material such as wax or apolycarbonate which will dissolve or decompose in a solvent is useful.If the interposer structure will remain, any structurally appropriatematerial which can withstand the fabrication and operating temperaturesof the assembly can be used. Examples include printed circuit boards andhard or hard filled plastics such as polyphenylene sulfide.

FIG. 18 is a schematic side view of interface structures 134 andinterposer structure 132. In one embodiment, rivet-like shapes of theinterface structures are formed so that portions of the interfacestructures extend over the interposer structure to prevent the interfacestructures from falling out of the interposer structure. The rivetshapes can be formed in any conventional manner such as by pressing, forexample.

A loose fit rivet 134a which can move horizontally and/or vertically canbe useful for reducing stress on the interposer structure and contactareas during high temperature operations, providing flexibility forsolder to move interfaces in alignment with contact pads, and providingflexibility for curved surfaces. In one embodiment for creating a loosefit, the interposer structure is dipped in a material that can bedissolved in a solvent, the interface structures are inserted in theinterposer structure, the rivets are created, and the interposerstructure is dipped in a solvent for removing the dipped material.

As discussed with respect to interface structures 126 and 128 of FIG.16, the interface structures can be modified by filling them with aninsulative substance that prevents solder wicking into the crushed orvertically stranded wires. If the solder wicks deeply enough into theinterfaces, a solid inflexible structure can result and would notprovide effective CTE thermal stress accommodation.

The insulative material may be added by a method such as dipping andcomprise a polymer that can be dissolved in an appropriate solvent orany of a variety of organic and inorganic materials that can be removedby any of a number of techniques. A heat sublimation technique isdescribed in Wojnarowski et al., U.S. Pat. No. 5,449,427, issued Sep.12, 1995, for example.

After assembly, the insulative material may be removed, if desired, byany of a number of techniques such as sublimation, solvents, thermaldecomposition, and the like to increase the spring-like compliance ofthe interface structures.

FIG. 19 is a view similar to that of FIG. 18 further includingsolderable surfaces 138 on the interface. The top and bottom areas ofthe interfaces can be chemically treated or mechanically ground toreveal a solderable interface. Plasmas, RIE (reactive ion etching),solvents, flame treatments, or mechanical grinding, for example, can beused to expose desired areas of the rivet buttons. An optional step ofpre-tinning an interface structure surface by dipping it in solder ormetallizing it by electroless metallization can facilitate soldering ofthe interface surface to the contact area and help the surface tensionof the solder to move the interfaces into alignment with the contactpads.

Repair can be accomplished by heating the interfaces and melting thesolder. If no insulating material is present (either there never was anyor any such material was removed), the solder wicks away from the solderjoint and into the interface structures and thus the excess solder isremoved from the repair area without requiring rework by hand.

The present invention can be used as a planarization technique toassemble non-flat parts. This is useful because surfaces such as PCboards are not always flat and because surfaces to be interconnected maybe curved, for example. FIG. 20 is a view similar to that of FIG. 17showing a curved base 310 with pads 312 coupled to pads 318 of surface316 by interfaces 334 loosely fit into an interposer 332.

While only certain preferred features of the invention have beenillustrated and described herein, many modifications and changes willoccur to those skilled in the art. It is, therefore, to be understoodthat the appended claims are intended to cover all such modificationsand changes as fall within the true spirit of the invention.

What is claimed is:
 1. An interface comprising:a base surface having anelectrically conductive base pad; a compliant coating over the basesurface and base pad, the compliant coating having a coating viaextending therethrough to the base pad; base metallization patternedover the compliant coating and extending into the coating via; a lowmodulus dielectric interface layer overlying the compliant coating andhaving at least one dielectric interface via extending therethrough tothe base metallization; and a floating pad structure comprising floatingpad metallization patterned over the dielectric interface layer, a firstportion of the floating pad metallization forming a central pad and asecond portion of the floating pad metallization forming at least oneextension from the central pad and extending into the at least onedielectric interface via, the floating pad structure capable ofaccommodating thermal and material stresses and permitting movements ofthe central pad independent of the base pad.
 2. The interface of claim 1wherein the base surface comprises a semiconductor chip, a passivecomponent, a semiconductor wafer, a printed circuit board, a flexibleinterconnect layer structure, a photonic structure, a liquid crystalstructure, or a microelectromechanical structure (MEMS).
 3. Theinterface of claim 2 wherein the dielectric interface layer comprises aflexible epoxy or a flexible rubber.
 4. The interface of claim 1 whereinthe floating pad metallization includes at least two extensions from thecentral pad and wherein the dielectric interface layer has at least twodielectric interface vias therein, each extension extending into arespective dielectric interface via.
 5. The interface of claim 4 whereinthe central pad has a thickness greater than a thickness of the at leasttwo extensions.
 6. The interface of claim 4 wherein, above the base pad,the levels of the dielectric interface layer and the floating padmetallization dip.
 7. The interface of claim 4 wherein the floating padmetallization further includes at least two noncoupling extensions, theat least two noncoupling extensions not extending into dielectricinterface vias; andfurther includingan additional dielectric interfacelayer overlying the floating pad structure and having additional viasextending therethrough to the at least two noncoupling extensions; andan additional floating pad structure comprising additional floating padmetallization patterned over the additional dielectric interface layer,a first portion of the additional floating pad metallization forming anadditional central pad and a second portion of the additional floatingpad metallization forming additional extensions from the central pad andextending into respective ones of the additional vias.
 8. The interfaceof claim 4 wherein a portion of the surface not including the base padand the floating pad structure is not covered by the compliant coatingand the dielectric interface layer.
 9. The interface of claim 4 whereina solder ball or solder bump is attached to the central pad of thefloating pad structure.
 10. The interface of claim 4 furtherincluding:an additional dielectric interface layer overlying thefloating pad structure and having an additional via over the centralpad, and contact metallization patterned over the additional dielectricinterface layer and extending into the additional via.
 11. The interfaceof claim 10 further including a solder ball or solder bump attached tothe contact metallization.
 12. The interface of claim 4 furtherincluding:an additional dielectric interface layer overlying thefloating pad structure and having an additional via over the centralpad, and solder patterned over the additional dielectric interface layerand extending into the additional via.
 13. The interface of claim 12wherein the additional dielectric interface layer comprises a soldermask.
 14. The interface of claim 12 further including a secondadditional interface dielectric layer overlying the additional interfacedielectric layer and having second additional vias over the solder. 15.The interface of claim 4, wherein the interface dielectric layer has anopen area situated under at least a portion of the floating padstructure.
 16. An interface comprising:a base surface having anelectrically conductive base pad; a compliant coating over the basesurface and base pad, the compliant coating having a coating viaextending therethrough to the base pad; base metallization patternedover the compliant coating and extending into the coating via; afloating pad structure situated above the base metallization andcompliant coating and comprising floating pad metallization, a firstportion of the floating pad metallization forming a central pad and asecond portion of the floating pad metallization forming at least twoextensions from the central pad, the at least two extensions beingcoupled to the base metallization, the floating pad structure capable ofaccommodating thermal and material stresses and permitting movements ofthe central pad independent of the base pad.
 17. A method forfabricating an interface comprising the steps of:applying a compliantcoating over a base surface having an electrically conductive base pad;forming a coating via extending in the compliant coating extending tothe base pad; applying and patterning a base metallization over thecompliant coating and into the coating via; applying a low modulusdielectric interface layer over the compliant coating; forming at leastone dielectric interface via extending through the dielectric interfacelayer to the base metallization; and applying a floating pad structurecomprising floating pad metallization patterned over the dielectricinterface layer, a first portion of the floating pad metallizationforming a central pad and a second portion of the floating padmetallization forming at least one extension from the central pad andextending into the dielectric interface via, the floating pad structurecapable of accommodating thermal and material stresses and permittingmovements of the central pad independent of the base pad.
 18. The methodof claim 17 wherein the at least one dielectric interface via comprisesat least two dielectric interface vias and the at least one extensioncomprises at least two extensions and further including, after applyingthe floating pad structure, removing the dielectric interface layer. 19.The method of claim 17 wherein the at least one dielectric interface viacomprises at least two dielectric interface vias and the at least oneextension comprises at least two extensions and further including, afterapplying the floating pad, removing at least a portion of the dielectricinterface layer under the floating pad structure.
 20. The method ofclaim 19 further including the step of filling an area under thefloating pad structure wherein the at least a portion of the dielectricinterface layer was removed with a filler material.
 21. An interfacecomprising:a substrate including a low modulus dielectric interfacematerial having at least one hole extending at least partiallytherethrough; and a metallized floating pad contact structure comprisingelectrically conductive material coating the at least one hole, at leastsome of the metallized floating pad contact structure forming at leastone extension from the at least one hole.
 22. The interface of claim 21further comprising:an electrically conductive pin connected to a supportstructure and situated in the at least one hole, and solder coupling theelectrically conductive pin and the floating contact structure.
 23. Theinterface of claim 22 wherein the substrate further includes a rigidmaterial surrounding the low modulus dielectric interface material. 24.The interface of claim 22 wherein the substrate comprises a printedcircuit board or a flexible interconnect layer structure.
 25. Theinterface of claim 24 wherein the dielectric interface materialcomprises a flexible epoxy or a flexible rubber.
 26. The interface ofclaim 21 wherein the floating pad metallization includes at least twoextensions from the central pad.
 27. The interface of claim 21 whereinthe at least one hole extends partially through the substrate.
 28. Theinterface of claim 27 wherein the substrate further comprises substratepad metallization situated on an opposite surface of the at least onehole and further comprising:an electrically conductive pin connected toa support structure and situated in the at least one hole; soldercoupling the electrically conductive pin and the floating contactstructure; a surface having a surface metallization pad thereon; andsolder coupling the substrate pad metallization pad and the surfacemetallization pad.